Descriptions

Two-year-old Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings of two seed sources raised in three nurseries in Oregon and Washington were tested for differences in frost hardiness from September 1985 to March 1986. The objective of the study was to determine whether nursery location had an influence on seedling acclimation, deacclimation , budburst and first-year field performance.
Frost hardiness was determined five times from September to December in the nurseries. In January 1986 seedlings were lifted at the three nurseries. Dehardening of potted trees was observed under outdoor and growth chamber conditions. Budburst of the trees dehardening outdoors was recorded from March to June. Hardiness was determined with a whole plant freezing test. Seedlings were frozen at each sampling date to 4 test
temperatures to evaluate needle, bud and stem tissue damage. In general, trees raised in the highest elevation nursery or the most northern nursery had hardier tissue than seedlings raised in the coastal nursery. However, hardiness varied for each tissue and among nurseries and seed sources. In February and March seedlings from the Cascade (975m) seed source were less hardy than seedlings from the coastal source (450m). A growth chamber experiment confirmed the outdoor dehardening pattern.
With a 16 hour photoperiod a constant temperature of +5°C maintained cold hardiness, whereas +10°C and 15°C promoted rapid dehardening after 20 days. To predict hardening, a regression equation with nursery
weather data and elevation was calculated. Photoperiod, number of frost days, and elevation were the most important independent
factors predicting hardening (R²=0.29).
The environment of the three nurseries seemed to have a strong influence on budburst. Trees raised in the coastal nursery
burst bud significantly earlier than trees from the other two nurseries. Mean terminal budburst of potted trees from both seed
sources in each nursery occurred only 2 days apart. Frost hardiness in January and first-year field growth were not
correlated. A correlation between the electrolytic conductivity of shoot tips and the damage of needles, buds, and stems as determined by the whole plant freezing test was poor. The highest correlation coefficient (r) was 0.69. Different hardening rates of tissues and plant to plant variability may have contributed to the low correlation. The conductivity method as outlined in this study does not provide a satisfactory estimation of frost hardiness of
two-year-old bareroot Douglas-fir seedlings.